Short wave radiation, primarily in the visible range of the spectrum, will penetrate our atmosphere and be reflected or absorbed by the surface of the earth (water or land). If it is reflected it will pass back out of our atmosphere and will NOT warm the earth as part of the Greenhouse Effect.
If the short wave radiation is ABSORBED it must be re-radiated. But, because the earth is so cold compared to the sun, it will be RE-RADIATED as IR, and IR will not pass directly out of the atmosphere (i.e., the wavelength has been converted). This re-radiated IR from the earth's surface will be absorbed by the gases in the atmosphere which, in turn, will re-radiate it also as IR, with some of that radiation being directed back toward the earth.
This temporary "retention" of short wave radiation from the sun (by absorption and re-radiation, thereby changing its wavelength to IR) is the key concept of the Greenhouse Effect.
Note, finally, that the solar radiation initially absorbed by the gases in the atmosphere ALSO WILL CONTRIBUTE to the Greenhouse Effect, because the gases will also re-radiate IR.
Conductive heat flows down a gradient from hot to cold.
If the air is colder than the water across the air-sea BL, Heat will flow via conduction from water to the air:
• The air BL will increase in temp. and become less dense, establishing unstable conditions
• Warmer, less dense air rises, creating vertical convection currents
• Cooler more dense water lost from surface sinks and is replaced by 30 C water from below, because the loss of heat from the water BL, which decreases the temp. and increases the water BL density, creating unstable conditions and vertical convection.
If the air is warmer than the water, the heat will flow via conduction from the air to the water:
• The air BL will decrease in temp. and become more dense, creating stable conditions and no convection currents in the air
• The gaining of heat in the water BL causes the temp. to rise and become less dense, stable conditions are established, no convection current created